The emergence of low-power and low-cost sensing technologies enables the development of electromagnetic tags. A tag has a unique ID that is remotely readable via an electromagnetic channel by a tag reader. When attached to consumer goods, the tags can be used as identifiers that may replace Uniform Product Codes (Bar Codes) in the near future. It is because the tags, unlike the bar codes, can be read without direct line of sight. The tags can also be implemented on smart automated systems that exhibit intelligent, collaborative behavior, which may bring drastic improvements on human productivity and efficiency. Example applications of the tags include automatic object tracking, inventory and supply chain management, and Web appliances.
A tag system is similar to a multi-access communication system in that tags and the tag reader share a bandwidth-limited channel. To resolve conflicts in accessing the channel, the tag system uses a collision resolution protocol that requires no prior scheduling or central control, in a similar manner as in a multi-access system. However, additional challenging issues exist in a tag system. Because of severe cost constraint in a tag system, the tags are very limited in memory, power, and computations capabilities. Because of the limited resources available in the tags, it is not desirable for the tags to communicate with each other directly or to maintain dynamic states in their circuitry.